Under bump metallurgy for Lead-Tin bump over copper pad

ABSTRACT

The present invention describes a method including providing a component, the component having a bond pad; forming a passivation layer over the component; forming a via in the passivation layer to uncover the bond pad; and forming an under bump metallurgy (UBM) over the passivation layer, in the via, and over the bond pad, in which the UBM includes an alloy of Aluminum and Magnesium.  
     The present invention also describes an under bump metallurgy (UBM) that includes a lower layer, the lower layer including an alloy of Aluminum and Magnesium; and an upper layer located over the lower layer.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to the field of semiconductorintegrated circuit (IC) manufacturing, and more specifically, to amethod of forming a more reliable under bump metallurgy (UBM) and an UBMthat is more reliable.

[0003] 2. Discussion of Related Art

[0004] Chip-to-package interconnections have traditionally involvedwirebonding which is very cost-effective. Wirebonding is the use of veryfine metal wires to join contacts on a chip with corresponding contactson a package. As the size of a transistor is reduced, the size of thechip-to-package interconnection also has to be scaled down. However, theperformance and the reliability of the chip-to-package interconnectionmay be affected since wirebonding requires the routing of all theinput/output (I/O) connections to the edges of the chip.

[0005] Solder bumping is the use of reflowable solder balls to joincontacts on the chip with corresponding contacts on the package. Solderbumping requires that the chip be flipped over to face the package.Solder bumping permits I/O connections to be placed across the surfaceof the chip, which results in many advantages. First, bumpingsignificantly increases the density of the I/O connections. Second,bumping simplifies the design and layout of the chip. Third, bumpingdecreases the footprint of the package. Fourth, bumping greatly enhancesthe reliability of the I/O connections.

[0006] Transistors on the chip have traditionally been connected withAluminum lines. As the size of the transistor continued to be reduced,Copper was introduced as a replacement for Aluminum. Copper has lowerresistivity than Aluminum so performance of the chip is improved. Copperis more resistant to electromigration than Aluminum so reliability ofthe chip is also improved.

[0007] The bump attached to the bond pad of the chip has traditionallybeen formed with a Lead-Tin Solder. However, during thermal cycling, theTin in the bump tends to migrate through cracks or other defects in theUBM and react with the Copper in the bond pad to form an intermetalliccompound which may cause shorting of the interconnects thereby leadingto premature failure of the interconnect.

[0008] Thus, what is needed is a method of forming a more reliable underbump metallurgy (UBM) and an UBM that is more reliable.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIGS. 1A-1E are illustrations of an elevation view of anembodiment of a method of forming a more reliable under bump metallurgy(UBM) according to the present invention.

[0010]FIG. 1E is also an illustration of an elevation view of a morereliable UBM, according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0011] In the following description, numerous particular details, suchas specific materials, dimensions, and processes, are set forth in orderto provide a thorough understanding of the present invention. However,one skilled in the art will realize that the invention may be practicedwithout these particular details. In other instances, well-knownsemiconductor equipment and processes have not been described inparticular detail so as to avoid obscuring the present invention.

[0012] The present invention includes a method of forming a morereliable under bump metallurgy (UBM) and a UBM that is more reliable.The method of the present invention suppresses the diffusion of Tin froma solder bump to an underlying Copper bond pad, minimizes the formationof a Copper-Tin (Cu:Sn) intermetallic compound, and prevents shorting ofinterconnects. The UBM of the present invention may include Aluminumwith an alloying element such as Magnesium.

[0013] An embodiment of a method of the present invention is shown inFIGS. 1A-1E. A component 100 may include a substrate 105, in which adevice, such as a transistor, has been formed from a semiconductingmaterial, such as Silicon, an insulating material, such as Silicon Oxideand Silicon Nitride, and a conducting material, such as dopedPolysilicon and Copper.

[0014] The transistor in the substrate 105 may include interconnectsthat have been formed from multiple layers of conducting lines which areisolated by insulating material. The conducting lines on the differentlayers may be connected by conducting plugs in vias formed through theinsulating material. The conducting lines and plugs may be formed fromthe same or different materials, such as Copper metal or alloy. Theinsulating material may be an interlayer dielectric (ILD) formed fromSilicon Oxide or a low dielectric constant material, such as a porousCarbon-doped Oxide (CDO or SiOC).

[0015] A landing pad, or bond pad 110, may be formed over the substrate105 to provide access to the interconnects of the underlying device. Thebond pad 110 may permit Input/Output (I/O) of an electrical signal,power, or ground, to and from the underlying device, such as thetransistor, through the interconnects. The bond pad 110 may be formedfrom Copper metal or alloy.

[0016] As shown in an embodiment of the present invention in FIG. 1A, apassivation layer 120 may be formed over the substrate 105 to keep outcontaminants and moisture and prevent corrosion or other damage to theinterconnects and the underlying device. The passivation layer 120 mayalso serve as a planarizing layer to assist in subsequent processing,especially photolithography. The passivation layer 120 may further serveas a stress buffer layer. The characteristics that are desired for thepassivation layer 120 include good adhesion, good thermal stability,high tensile strength, and good chemical resistance.

[0017] The passivation layer 120 may include Silicon Oxide, SiliconNitride, and an organic material, such as a polyimide. In oneembodiment, the polyimide may be covered with a radiation-sensitivematerial, such as a photoresist, and patterned with photolithography. Inanother embodiment, a photodefinable polyimide, may be spin-coateddirectly over the bond pad 110 and the substrate 105. The photodefinablepolyimide may be exposed to radiation of the appropriate wavelength,energy, and dose, as modulated by a mask with a bump pattern. Developingof the photodefinable polyimide followed by etching to uncover the bondpad 110 will form a via 125 over the bond pad 110, as shown in anembodiment of the present invention in FIG. 1B.

[0018] In order to prevent high contact resistance, a plasma pre-clean,or ashing, may be performed to remove any Oxide that may be present onthe bond pad 110. Then, an UBM 130 may be formed over the passivationlayer 120, in the via 125, and over the bond pad 110, as shown in anembodiment of the present invention in FIG. 1C.

[0019] The UBM 130 provides a reliable electrical and mechanicalinterface between the underlying bond pad 110 and the overlying bump155. In an embodiment of the present invention, the UBM 130 may includea multilayer stack of materials.

[0020] In one embodiment of the present invention, the UBM 130 mayinclude an upper layer 136 located over a lower layer 133. In anembodiment of the present invention, the upper layer 136 of the UBM 130may be formed from a Nickel-Vanadium (NiV) alloy which is wettable bythe solder 150. A material that is wettable by the solder 150 willdissolve in the solder 150 to strengthen the metallurgical bond. In anembodiment of the present invention, the upper layer 136 of a NiV alloymay have a thickness of about 50-1,000 nm.

[0021] In another embodiment of the present invention, the lower layer133 of the UBM 130 may be formed from a stack of materials, including anAluminum alloy that is sandwiched between a top Titanium and a bottomTitanium. The top Titanium improves adhesion between the upper layer 136and the lower layer 133 of the UBM 130. The top Titanium also suppressesdiffusion of Nickel (Ni) from the upper layer 136. In an embodiment ofthe present invention, the top Titanium may have a thickness of about20-500 nanometers (nm).

[0022] The Aluminum alloy in the stack may include Aluminum and one ormore suitable alloying elements. An alloy is a solid solution of two ormore metals. A suitable alloying element will suppress the diffusion ofTin from the overlying bump 155. In an embodiment, the suitable alloyingelement may be about 0.5-2.5% Magnesium by weight. In other embodimentsof the present invention, the suitable alloying element may includeChromium (Cr), Germanium (Ge), Hafnium (Hf), Lithium (Li), Manganese(Mn), Palladium (Pd), Vanadium (V), and Zirconium (Zr). In general,Titanium (Ti) and Silicon (Si) are not suitable alloying elements forthe Aluminum. In one embodiment of the present invention, the Aluminumalloy may have a uniform composition through its thickness. In anotherembodiment of the present invention, the Aluminum alloy may have agraded composition through its thickness. In one embodiment of thepresent invention, the Aluminum alloy may have a thickness of about100-1,000 nm. In another embodiment of the present invention, theAluminum alloy may have a thickness of about 100-400 nm.

[0023] In one embodiment of the present invention, the Aluminum alloysuppresses diffusion of Tin (Sn) from the overlying bump 155. In anotherembodiment of the present invention, the Aluminum alloy suppressesdiffusion of Copper (Cu) from the underlying bond pad 110. The formationand growth of Copper-Tin (Cu:Sn) intermetallic compounds is controlledand limited to prevent shorting of interconnects.

[0024] The bottom Titanium improves adhesion between the lower layer 133of the UBM 130 and the underlying bond pad 110. Titanium decreasesinterfacial contact resistance to the bond pad 110 by chemicallyreducing any oxide that may be present on the bond pad 110. The edges ofthe via 125 formed through the passivation layer 120 should behermetically sealed by the UBM 130 to prevent corrosion of theinterconnects that are located below the bond pad 110. Titanium alsoincreases resistance to electromigration. In an embodiment, the bottomTitanium may have a thickness of about 20-500 nm.

[0025] The UBM 130 may be formed as a blanket film by physical vapordeposition (PVD) or sputtering. In one embodiment of the presentinvention, the UBM may be formed by ionized PVD (I-PVD) to achieve goodconformality in filling a via 125 having a high aspect ratio.

[0026] If two or more layers are being sputtered sequentially for theUBM 130, a different target may be used for each layer. The sequentialsputtering may be done without breaking vacuum so as to reducecontamination, prevent formation of Oxides, and improve throughput.

[0027] If two or more materials, such as Aluminum and Magnesium, are tobe co-sputtered for a layer in the UBM 130, a particular composition ofthe sputtering target may be specified in order to produce the desiredcomposition in the layer of the UBM 130 being formed. The difference incomposition between the target and the layer may be caused by adifference in sputtering efficiency or sticking coefficient.

[0028] In another embodiment of the present invention, the compositionof a layer that has been sputtered may be modified by annealing. Theannealing may be performed in a gas. The gas may be inert or reactive.

[0029] Next, the UBM 130 may be covered with a layer of photoresist 140.After aligning the component 100 with respect to a mask, the photoresist140 may be exposed with the appropriate radiation. After developing thephotoresist 140, an opening 145 is formed in the photoresist 140 overthe upper layer 136 of the UBM 130, as shown in an embodiment in FIG.1D. In one embodiment, the opening 145 in the photoresist 140 may belocated over the bond pad 110. In another embodiment, the opening 145may be offset to one side of the bond pad 110.

[0030] An electroplating cell includes two electrodes that are immersedin an electrolyte and connected through an external circuit to a powersupply. A consumable anode in the electroplating cell may include analloy of the metals which form the solder 150. In one embodiment, thesolder 150 may be a high Lead solder, having a composition of 95% Lead(Pb) and 5% Tin (Sn), by weight. The external circuit may removeelectrons from the anode to oxidize the metals and releasepositively-charged metal ions into the electrolyte.

[0031] The UBM 130 may serve as a cathode in the electroplating cell.The UBM 130 provides a low-resistance electrical path for the externalcircuit to supply electrons to reduce the positively-charged metal ionsin the electrolyte and electroplate the metals, through the opening 145in the photoresist 140, over the upper layer 136 of the UBM 130.

[0032] The solder 150 will spread out in a mushroom shape once thethickness of the solder 150 being electroplated in the opening 145 ofthe photoresist 140 exceeds the thickness of the photoresist 140, asshown in an embodiment of the present invention in FIG. 1D.

[0033] In another embodiment of the present invention, a very thicklayer of photoresist 140 is used so the thickness of the solder 150being electroplated in the opening 145 of the photoresist 140 does notexceed the thickness of the photoresist 140. As a result, the solder 150retains a pillar shape within the opening 145 in the photoresist 140.

[0034] After the solder 150 has been electroplated into mushroom-shaped,or pillar-shaped, islands on the component 100, the photoresist 140 isstripped off.

[0035] In order to prevent shorting of the islands of electroplatedsolder 150, a wet etch solution, that will not etch the solder 150, maybe used, in one embodiment of the present invention, to etch away theexposed portion of the UBM 130 that is not covered by the solder 150.Etching around the edges of the covered portion of the UBM 130 locatedbeneath the islands of solder 150 should be controlled so any undercutis minimized, such as to 2 microns (um) or less.

[0036] After etching away the exposed portion of the UBM 130, theislands of solder 150 may be reflowed. The melting (liquidus)temperature of the solder 150 depends on the alloy composition of thesolder 150. A high Lead solder, such as 95 Pb/5 Sn by weight percent,may flow at about 308 degrees Centigrade.

[0037] A convection oven may be used to reflow the solder 150. Forminggas may be used as a cover gas in the convection oven. Forming gas mayhave a passive component, such as 90% Nitrogen to prevent formation ofOxides, and an active component, such as 10% Hydrogen to chemicallyreduce existing Oxides.

[0038] Upon cooling, surface tension will draw each island of solder 150into a bump 155 having an approximately spherical shape, as shown in anembodiment of the present invention in FIG. 1E. The minimum distancebetween the centers of adjacent islands of solder 150, or bump pitch, islimited by assembly and reliability considerations and may be selectedfrom a range of about 50-300 um.

[0039] After solidification, the bump 155 may have a diameter selectedfrom a range of about 20-150 um. The diameter and height of the bump 155depends on the area of the wettable metal base, which is determined bythe undercut of the UBM 130. The thickness, or bump 155 height, shouldbe well-controlled, with a standard deviation of less than 2.5 um withinthe component 100 and across a batch of components 100.

[0040] The bump 155 height will affect the standoff height when thecomponent 100 is later attached to a substrate in a package. Theuniformity of bump 155 height across the component 100 will also affectthe coplanarity of the bumps 155 across the component 100. Coplanarity,in turn, determines how reliably all of the bumps 155 on the component100 may be subsequently connected to the pads on the substrate in thepackage.

[0041]FIG. 1E also shows a more reliable UBM 130, according to anembodiment of the present invention. In one embodiment of the presentinvention, the UBM 130 may include a multilayer stack of materials, suchas a lower layer 133 and an upper layer 136.

[0042] In one embodiment of the present invention, the lower layer 133of the UBM 130 may include an Aluminum alloy. The Aluminum alloy mayinclude one or more suitable alloying elements, such as Magnesium. Thesuitable alloying elements suppress the diffusion of metals, such as Tin(Sn), from the overlying bump 155. In an embodiment of the presentinvention, the Aluminum alloy may include about 0.5-2.5% by weight ofMagnesium. In one embodiment of the present invention, the Aluminumalloy may have a uniform composition through its thickness. In anotherembodiment of the present invention, the Aluminum alloy may have agraded composition through its thickness. In one embodiment of thepresent invention, the Aluminum alloy may have a thickness of about100-1,000 nm. In another embodiment of the present invention, theAluminum alloy may have a thickness of about 100-400 nm.

[0043] In another embodiment of the present invention, the lower layer133 of the UBM 130 may include a stack of Titanium/Aluminumalloy/Titanium. The Titanium underlying the Aluminum alloy may have athickness of about 20-500 nm. The Titanium overlying the Aluminum alloymay have a thickness of about 20-500 nm.

[0044] The upper layer 136 of the UBM 130 may include a Nickel-Vanadium(NiV) alloy. The NiV alloy may have a thickness of about 50-1,000 nm.

[0045] Many alternative embodiments and numerous particular details havebeen set forth above in order to provide a thorough understanding of thepresent invention. One skilled in the art will appreciate that many ofthe features in one embodiment are equally applicable to otherembodiments. One skilled in the art will also appreciate the ability tomake various equivalent substitutions for those specific materials,processes, dimensions, concentrations, etc. described herein. It is tobe understood that the detailed description of the present inventionshould be taken as illustrative and not limiting, wherein the scope ofthe present invention should be determined by the claims that follow.

[0046] Thus, we have described a method of forming a more reliable underbump metallurgy (UBM) and an UBM that is more reliable.

We claim:
 1. A method comprising: providing a component, said componenthaving a bond pad; forming a passivation layer over said component;forming a via in said passivation layer to uncover said bond pad; andforming an under bump metallurgy (UBM) over said passivation layer, insaid via, and over said bond pad, said UBM comprising an alloy ofAluminum and Magnesium.
 2. The method of claim 1 further comprising:forming a photoresist over said UBM; forming an opening in saidphotoresist to uncover said UBM over said bond pad; forming a solderover said opening; removing said photoresist; removing an exposedportion of said UBM that is not covered by said solder; and reflowingsaid solder into a bump.
 3. The method of claim 1 wherein said alloy insaid UBM comprises about 0.5-2.5% by weight of Magnesium.
 4. The methodof claim 1 wherein said alloy in said UBM is formed by co-sputteringsaid Aluminum and said Magnesium.
 5. The method of claim 4 furthercomprising forming a bottom Titanium underlying said alloy in said UBM.6. The method of claim 4 further comprising forming a top Titaniumoverlying said alloy in said UBM.
 7. An under bump metallurgy (UBM)comprising: a lower layer, said lower layer comprising an alloy ofAluminum and Magnesium; and an upper layer disposed over said lowerlayer.
 8. The UBM of claim 7 wherein said alloy comprises about 0.5-2.5%by weight of Magnesium.
 9. The UBM of claim 7 wherein said alloycomprises a thickness of about 100-1,000 nanometers.
 10. The UBM ofclaim 7 wherein said lower layer further comprises a bottom Titaniumunderlying said alloy.
 11. The UBM of claim 7 wherein said lower layerfurther comprises a top Titanium overlying said alloy.
 12. The UBM ofclaim 10 wherein said bottom Titanium underlying said alloy comprises athickness of about 20-500 nanometers.
 13. The UBM of claim 11 whereinsaid top Titanium overlying said alloy comprises a thickness of about20-500 nanometers.
 14. The UBM of claim 7 wherein said upper layercomprises a Nickel-Vanadium alloy.
 15. The UBM of claim 14 wherein saidNickel-Vanadium alloy comprises a thickness of about 50-1,000nanometers.